With their increasing popularity around the world, scientists have started to investigate the evidence for claimed health benefits.

Researchers from Bristol University and the University of Eastern Finland studied a population of 1,935 men aged from 42 to 61 and tracked their health over a quarter of a century.

Researchers recorded how often each man used a sauna and then documented how many were admitted to hospital over the next 25 years for severe asthma attacks, pneumonia caused by chest infections and complications caused by chronic obstructive pulmonary disorder, or COPD.

This is a serious lung ailment often caused by smoking.

The results revealed 379 of the men needed hospital treatment for respiratory illnesses during the study period.

But those who had a sauna two to three times a week were 27 per cent less likely to fall ill than men who rarely or never used them.

Sauna fans enjoying four or more sessions a week were 41 per cent less likely to develop pneumonia.

Although it was a men-only study, the benefits are likely to apply to women too.

Pneumonia is the sixth biggest cause of death in the UK, claiming an estimated 29,000 lives a year.

Scientists are not sure why saunas have such a dramatic impact on respiratory illnesses but the theory is that the heat generated by them eases airway obstruction.